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Suddenly, a Sinkhole

The crew had already begun setting the casing and with 63 feet of it in place, they were getting ready to add another piece. The helper and welder for the project, Dale Holloway, had just gotten through welding the second joint and was preparing for the next step. Raising his hood, he had turned to the driller to indicate he was ready for the next section to be added when he felt something, as though the mud pan he was standing on had moved.

Craig Hall, the driller on the job, recalls what happened next: "So he stepped off of it, turned and looked back and when he did, [the rig] took off and fell six feet straight down. The whole back of the rig was level with the top of the dirt. If he had not moved, 30 seconds later, I may not have a helper. He took off running to get away from the machine, and over the next 15 minutes, it sunk another four or five feet into the ground."

With the machine having fallen straight backwards, the boom was misaligned by 40 feet, and the front tires of the truck were elevated 18 inches off the ground.

Once the rig began sinking, the crew sped into emergency mode. Although the customer had a crane, it was being used on a job, and while that crane could be broken down and brought to the site, it would take a couple hours. However, with the customer's help, Hall quickly located another crane in nearby Georgia. Although it was 5 o'clock in the evening, understanding the seriousness of the situation, a crew and crane were on their way within a matter of minutes. "In about an hour, they drove up," Hall recounts, "and in about two hours, we had it up and out of the hole."

Borrowing a 6-foot double-hook lifting cable from the customer, they attached the cable's hooks to either side of the pivot point on the boom where it connects to the truck and lifted it from there.

Hall believes that a cavity near the production well the crew was installing was disturbed by the drilling. When the formation began to move, it took the end of the rig with it.

After the rig was safely returned to an upright position on solid ground, speculation began about how it had fallen in the first place. The formation had been nothing unusual for Florida. "We had a soft sand," describes Hall, "but it did not have any water and was pretty stable. We drilled down and drilled into some soft material down at about 65, 70 feet, but everything seemed to hold open all right. Then we drilled right on down, hit a few little streaks of rock and drilled right into consolidated rock at about 110, 115 feet, then into a soft cavity -- it was soft material but didn't feel like an open hole -- and then right back into good, consolidated rock."

Ultimately, they came to the conclusion that it was a sinkhole, two or three feet off to one side from where they were drilling, that had caused the incident.

The crew had been setting the pipe through the back of the machine and when they began investigating, they noticed that a few feet from the pipe there was a round hole, approximately 18 inches to 20 inches in diameter and fairly deep.

Hall speculates that perhaps they had drilled right next to a cavity and with the mass of the rig and the impact from drilling, the formation started moving into it. "We evidently had shaken the ground enough that all that material started moving. We're into a five-year drought here. We thought maybe [the cavity] used to have water in it and it was just dry and when we disturbed it, it took off."

The end of Hall's CF-1250 Failing drill rig had sunk to such an extent that its front tires were lifted about a foot and a half off the ground.

He deduces that if they had drilled into the cavity, the formation would have moved into the hole they were making. Instead, the material was going off to one side of the machine, sliding two or three feet to the right. "I reckon the weight of the machine is what pushed the material out that way, off into that hole," Hall comments.

The actual hole came out to be about 10 feet to 11 feet deep, requiring six 15-yard dump trucks to fill it.

Looking back, Hall figures his helper was very lucky to move when he did. "It was very close," he remarks. "Another 30 seconds, and it would have probably pinched him between the mud pan and the back of the machine. If it hadn't cut his legs off, it would have just crushed him to death. It was one of the closest encounters."

All told, the crew from Hall's Pump and Well Service drilled and had to move three times in order to put in one production well. The first time they uncovered the sinkhole that nearly swallowed the rig. The second time they drilled, the crew encountered a large cavity and ended up abandoning the hole and back-filling it with cement. Moving 200 yards up above the customer's original production well, the team from Hall's Pump and Well Service actually had a cave-in there after they had made the well and were preparing to set the pump.

But, with the crisis averted and the well finally installed after three attempts, Hall is philosophical about sinkholes and cave-ins. "It's just when you drill next to a river or down here in this limestone, you just get 'em."

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